In ionographic devices such as that described by U.S. Pat. No. 4,524,371 to Sheridon et al. or U.S. Pat. No. 4,463,363 to Gundlach et al., an ion producing device generates ions to be directed past a plurality of modulation electrodes for deposit on an imaging surface in imagewise configuration. In one class of ionographic devices, ions are produced at a coronode supported within an ion chamber, and a moving fluid stream entrains and carries ions produced at the coronode out of the chamber. At the chamber exit, a plurality of control electrodes or nibs are modulated with a control voltage to selectively control passage of ions through the chamber exit ions directed through the chamber exit are deposited on a charge retentive surface in imagewise configuration to form an electrostatic latent image developable by electrostatographic techniques for subsequent transfer to a final substrate. The arrangement produces a high resolution non-contact printing system. Other inographic devices exist which operate similarly, but do not not rely on a moving fluid stream to carry ions to a surface.
One problem affecting the control of image quality in ionographic devices is known as "blooming". Blooming is an unavoidable phenomenon resulting from the effect of previously deposited ions or charge on the path of subsequent ions directed to the charge retentive surface. The problem is particularly noticeable when printing characters and edges of solid areas, resulting in character defects known as "rocking chair bottoms" (FIG. 1A), "undercutting" (FIG. 1B) and "trapezoids" (FIG. 1C), (with input bit maps shown in dashed lines). For an ionographic printing system where the source of the modulated ion stream is spaced approximately 10 mils from the imaging surface, and having an approximately 1100 volt image potential, the blooming artifacts noted may include a pixel displacement of 1-2 pixels distance, which in a 300 spot per inch printing system would be about 67 microns. While closer spacing of the ion stream source would reduce the effect, the spacing is limited by mechanical and printing requirement, including surface runout.